Demodulation equipment receiving a receive signal constituted by a carrier wave modulated by a base band signal commonly includes a local oscillator producing a local signal which is used to perform frequency transposition into base band. It is necessary for the frequency difference between the carrier wave and the local signal to be known accurately so as to be able to recover the base band signal. Although the frequency of the local oscillator is designed to be equal to the frequency of the carrier wave, in practice it is slightly different therefrom.
The invention is thus applicable where two frequencies to be compared are close together. This applies, in particular, to the digital cellular radiocommunications system specified by the Special Mobile Group (GSM) that constitutes a part of the European Conference of Posts and Telecommunications Administrations (CEPT), in which a mobile terminal receives a base band receive signal which is modulated on the principle of minimum displacement using a Gaussian filter. In this type of modulation known as GMSK (Gaussian Minimum Shift Keying) the base band signal is presented for transmission in the form of a succession of signals while being periodically examined at a modulation frequency that is constant, with a symbol having a phase which is any one of the multiples of .pi./2, ignoring inter-symbol interference.
Although this form of modulation thus has four phase states, it is used in the system under consideration to convey binary information, with the phase difference between two successive signals being restricted to +.pi./2 or -.pi./2. The binary base band signal is consequently frequently translated on reception so as to have only two phase states: 0 and .pi..
After such frequency translation, the base band signal is demodulated and sampled at a sampling frequency which is at exactly the same ratio relative to the local signal frequency as the modulation frequency relative to the carrier frequency.
The phase of a sample is constituted by the sum of the phase shifts due to the modulation (0 or .pi.) plus the synchronization phase shift which is proportional to the frequency difference that is to be evaluated. In addition, all of the samples have the same modulus.
Thus, one known way of evaluating the synchronization phase shift is to raise a sample to the square, thereby eliminating its modulation phase shift.
A method commonly employed consists in adding together the squares of a first series of samples to produce a first vector and then to repeat this operation for a second series having the same number of samples so as to produce a second vector. The angular difference between these two vectors divided by the time interval between the middles of these two series thus makes it possible to obtain the frequency difference.
This solution is very sensitive to noise and to inter-symbol interference.
An object of the present invention is thus to provide a circuit for evaluating the frequency difference between a local signal and a receive signal that is capable of taking up a plurality of states, the performance of the circuit being significantly improved both relative to noise and relative to inter-symbol interference, as can be seen from simulation results given below. The invention is applicable, for example, to correcting the frequency of a local oscillator.